1. Technical Field
The application relates to a cutting insert for a drill according to at least one possible embodiment disclosed herein. The application further relates to a cutting insert designed as a single or double-sided indexable tip. Such indexable tips have, for example, a triangular, tetragonal, such as a square, pentagonal or hexagonal outline. The edges located between the corners are formed as cutting edges. The indexable tip is arranged in the insert seat of a drilling tool or a cassette to be inserted into a drilling tool in such a way that one of the edges cuts during the drilling operation, while the other edges lie inactive in the insert seat. After the active cutting edge has worn out, the indexable tip is simply released from the insert seat and indexed such that one of the formerly inactive cutting edges is now active, i.e. the next drilling operations are carried out with it. This rotation of the indexable tip in the insert seat is effected until all edges of the indexable tip have been used up and thus the entire indexable tip has worn out.
2. Background Information
Background information is for informational purposes only and does not necessarily admit that subsequently mentioned information and publications are prior art.
Furthermore, it is known to arrange several indexable tips in a drilling tool. Often the indexable tips are made of carbides. When several indexable tips are applied, for example two or more, the indexable tips are arranged on the drill in such a way that they together form the major cutting edge of the drill. Configurations are also known in which the indexable tips are arranged on the major cutting edge with an offset to each other.
That means that with the indexable tips, it is possible to substitute one or several cutting edges of the drill through indexable tips.
When talking about cutting inserts in the following, the statements refer to a cutting insert with one cutting edge. Of course, the indications made with regard to the cutting insert can at any time also be applied to an indexable tip, which differs—as mentioned before—from the single-edged insert by the fact that one insert possesses several cutting edges at a time. Cutting inserts are known in which the cut is distributed at the drill-hole bottom to an even number of cutting edges. With these tools, half of the cutting edges machine the drill-hole bottom in the area of the drill-hole center, whereas the other half of the cutting edges machine the area of the drill-hole bottom which faces towards the outer diameter. This distribution of cut leads to a division of the cutting forces. In this way, an inner lip and an outer lip are formed on each cutting insert. These inner and outer lips may be located at an angle to each other, i.e. they may form an angular cutting edge comprising an inner lip and an outer lip. For certain applications, a degenerated angular cutting edge can also be used, in which the outer and inner lips are oriented to each other at an angle of 180°. In this case, the cutting insert is arranged in the insert seat in such a way that the cutting edge obliquely engages the workpiece to be machined.
Ideally, the inner and outer lips are proportioned relative to each other in such a way that a unilateral drifting of the tool relative to the drilling axis is restricted or minimized. Due to the work tolerances inevitable in tool manufacture, it is, however, hardly possible in practice to manufacture a cutting insert in such a way that the above-mentioned unilateral drifting of the tool is completely excluded. The use of several cutting inserts entails, furthermore, the problem that the tolerance faults, in the worst case, add up, resulting in an increased tendency to drifting of the tool. Due to the antipenetration cutting force component, which is then effective, the drilling tool runs in an unstable manner during application, which will result in a poor drill-hole quality and an early failure of the tool.
It is true that the described angular or roof-shaped arrangement of inner and outer lips relative to each other already improves the drifting behavior. To minimize or restrict the drifting of the tool, it is often desirable to keep the angle formed by the inner and outer lips as acute as possible. This entails, however, an unfavorable chip shape. Furthermore, it enlarges the engagement length of the cutting edge, which, in turn, leads to an undesired increase of the cutting forces and to an enlargement of the depth of the first cut. From DE-T-694 16 248, an indexable tip is known, in which the outer and the inner lips are divided into two partial lips with different angles, so that on this cutting insert, pre-lips are arranged in front of the cutting edges properly speaking, in the manner of a plunging tool. However, this arrangement of the cutting edges results in an undesired chip shape, namely ribbon chips.